Ultrafast Protonation of an Amide: Photoionization-Induced Proton Transfer in Phenol-Dimethylformamide Complex Cation

In this study we explore the photoionization-induced proton transfer (PT) dynamics in the hydrogen-bonded complex of phenol (PhOH) and a simple amide, dimethylformamide (DMF). Neutral PhOH-DMF complexes produced in a supersonic expansion are photoionized by femtosecond 1 + 1 resonance-enhanced multiphoton ionization via its S₁ state, and the subsequent PT dynamics occurring in the [PhOH-DMF]⁺ cation is probed by delayed pulses that lead to ion fragmentation. The experiments and density functional theory calculations reveal that the photoionization-induced PT proceeds in two consecutive steps of very different time scales. Upon femtosecond ionization the [PhOH-DMF]⁺ cation is initially prepared with a non-PT geometry close to that of the dominant neutral complex. The ionic system then rapidly relaxes into a configuration possessing both non-PT and PT characteristics in ∼0.5 ps. This partial-PT intermediate then undergoes a much slower barrier crossing in ∼25 ps to a more stable structure in which PT is more complete. The slow isomerization step not only corresponds to PT but also to a hydrogen-bonding site switching. The present study simulates a scenario of suddenly bringing a strong acid to the close vicinity of an amide to watch how protonation occurs. Our results suggest that the initial protonation of a peptide-like unit in acid-induced protein processes requires a relaxation time of ∼0.5 ps, which must be taken into account in complete descriptions of protein dynamics.